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MECHANICAL SEPARATION                            271
kind ha,ve been in use in other industries for a much longer time. A classifier of the Federal-Esperanza type to be described later has been used for a long time in the manufacture of white lead for separating the uncorroded portion of the buckles following the corroding and grinding operations.
The three principal classifiers employed in ore milling are the Federal-Esperanza, Dorr and Akins, illustrated by Figs. 5, 6 and 7. The Federal-Esperanza machine consists of one or two sprocket chains passing around sprocket wheels and to which metal flights are secured. The material fed enters at the lower end and the slime overflows at the same point. The sands are dragged by the flights above the water
FIG. 6.—Dorr classifier.
in the settling tank in a launder extension, affording water and slime the opportunity to drain back and are discharged at the. upper end. The Federal-Esperanza type may be designated as a homemade machine. It cannot be built more cheaply than the patented Dorr and Akins machine and on fine material the Federal-Esperanza does not do so good work as the patented machines but it can treat larger material. The writer has used them on rock material up to a size which would just pass a 10-in. opening. The use with the 10-in. material was for loosening clay coats preparatory to spray washing. The rock fragments had stood in dumps outdoors for nearly a half century and had accumulated clay coats which did not soften completely after being submerged 25 to 30 min. in cold water. In water of 95°C. the. clay coats were thoroughly softened in 1 or 2 min. The period of 1 to 2 min. represented the time the material was in the classifier. The classifier tank was built of reinforced concrete. One very serious disadvantage of the Federal-Esperanza is the excessive wear of the chain pins and bushings if gritty material is being handled. The figure shows a light classifier with a single pair of main sprocket wheels. The principal designing point to be observed with these classifiers is to be certain that the larger main sprocket wheels at the bottom shall be of sufficient diameter so that the bearings can be placed above the water. The capacity per square inch of flight can be reckoned at 0.005 cu. ft. of dry material. The rate of travel of the chains and flights ranges from 15 to 30 ft. per minute.
The Dorr classifier uses a flight-pulling mechanism. On the upward or push motion of the blades they dip into the material which has settled in the enclosing tank and on the return motion they are raised above the bed of material being advanced. The back and forth motion of the rakes is given by a bell crank. The